1. Field of the Invention
This invention relates broadly to electro-optical gaging methods and systems. More particularly, this invention relates to an electro-optical method and system for gaging dimensions of an object and correcting for system nonlinearities. The invention may be used to determine a lateral dimension of a moving hot bar during bar rolling in a steel mill as is disclosed herein. Similarly, the invention may be used to gage the dimensions of other shaped objects and in other environments as well.
2. Description of the Prior Art
Generally, in steel mills where hot round bars are rolled, productivity demands require that a variety of bars be rolled at speeds of up to 4000 ft./min. (1219 m.) and sizes of up to three inches in diameter (7.62 cm.) while the bar rolling temperature is about 1700.degree. F. (930.degree. C.). Further demands require that the specifications on finished cold bar size and out-of-roundness be within one-half existing commercial tolerances. In order to meet these requirements, a computer-controlled rolling process must be implemented that will combine order data with operating measurements to produce mill control signals that will maximize productivity while minimizing, or desirably eliminating, off-specification product.
Some of the operating data used in mill control computer calculations and referred to herein are: desired bar diameter, or aim size; aim size full- and half-commercial tolerances; and bar grade, or percent carbon composition of the bar to be rolled. One of the operating measurements mentioned above and of particular importance is actual bar diameter, or bar size. Another operating measurement is bar temperature, a parameter used to correct for hot bar shrinkage in both bar measurement and computer control aspects of mill operation.
In order that the mill control computer may be programmed to meet the strict requirements of mill speed, bar size and size half-tolerances, it is desirous that all operating measurements have the following characteristics. Bar size measurements be made when the bar vibrates in a lateral orbit while moving longitudinally during rolling; be made at repetitive rates of about 300 Hz.; have a resolution of 0.0005 inch; have an absolute accuracy equivalent to one-quarter commercial tolerance; maintain a high degree of reliability; all measurements made under the severe environment normally present in a steel rolling mill. Bar temperature measurements should have similar characteristics.
Several types of electro-optical gaging systems are available to measure bar size. One early type of bar size gaging system operates on the self-illumination principle in which chopped infrared radiation from the hot bar is imaged through a lens onto an infrared detector. Elementary edge-detection circuitry was used in an attempt to define raw detector pulses in relation to bar edges.
Three more recent electro-optical systems applicable to bar size measurements operate on the principle of back-lighting a test object to be measured and imaging a shadow of the object through a lens onto the face of an electronic camera. In one such gaging system, a scanning laser beam illuminates the test object and the lens system focuses the object shadow onto a phototransistor. In a second such gaging system, a stationary light source of fixed intensity illuminates the test object and the lens system focuses the object shadow onto an electronically scanned image orthicon tube having two-axis unidirectional scanning. In the third such system, the image orthicon tube is replaced by a self-scanning photodiode array.
The photoresponsive device in each of the three back-lighted gaging systems generates a raw camera pulse having a width that approximates the object dimension between shadow edges. Raw camera pulses are processed in edge detection circuitry having either plain differentiators or gated differentiators which further attempt to more closely define camera pulse width in relation to the object dimension.
Each of the foregoing prior art electro-optical bar size gaging systems has met with varying degrees of success in certain types of installations. However, none of these gaging systems is entirely satisfactory to use as a bar dimension gaging system in the environment of a contemporary high-speed hot steel bar rolling mill. Such gaging systems fail to meet the foregoing measurement requirements for one or more of the following reasons.
Difficulties with prior art gaging systems are first, the object to be measured must be confined to a given position in the camera field-of-view. Second, inability to provide sufficient camera speed-of-response and/or camera resolution. Third, inability to meet system accuracy at high repetition rates because considerable switching noise occurs at such measuring speeds and differentiator noise is also particularly troublesome. In addition, some environmental electrical noise is present in varying degrees which further compounds the problem of making definitive bar measurements at high speeds and high reliability. Fourth, inability or insufficient capability to correct for such error sources as optical and electronic nonlinearities, all of which affect gaging system accuracy. Fifth, instability which causes drift in system calibration. Sixth, inability to compensate or correct for distortion resulting from high frequency lateral vibration of the bar.